It has been hitherto publicly known to compound carbon fiber into plastics to obtain composite materials. It has also been well known that the thus obtained composite material shows improved properties in mechanical strength such as tensile strength and bending strength and heat-resistance. However, no improvement has been observed in its electrical properties.
On the other hand, the improvement of the electrical properties of a composite material has been achieved by compounding electroconductive carbon black into a plastic material. In these cases, it has been found that polypropylene provides better electrical properties then polyvinyl chloride, polystyrene and polyethylene. However, a composite material of polypropylene and electroconductive carbon black does not have sufficient in mechanical strength.
Accordingly, it has been considered that both the mechanical strength and the electrical properties of the composite material could be improved by compounding carbon fiber and electroconductive carbon black into plastics. Although such a composite can be achieved easily on an experimental scale, the incompatability of the plastics with carbon fiber and carbon black during compounding caused a reduction in the strength of the composite material and the stability of the molded composite material was sufficient over a long time period.
Accordingly, an object of the present invention is to provide a composite polypropylene resin material having good electrical properties and sufficient mechanical strength that can be easily compounded and readily molded without degradation of its strength and electrical properties.
In accordance with the present invention, this is achieved by compounding with the polypropylene, carbon fiber and electroconductive carbon black, a modified polypropylene in an amount of from 10-50 parts by weight based on 100 parts by weight of polypropylene obtained either by chemically modifying polypropylene with an organic unsaturated carboxylic acid or by copolymerizing polypropylene with the carboxylic acid.